Alarm system sensitive to cut cables and insensitive to r-f interference

ABSTRACT

IN AN ALARM SYSTEM WITH A BALANCED SHIELDED MICROPHONE CABLE A BARE DRAIN WIRE IS TIED TO THE CABLE SHIELD AT THE END OF THE MICRPHONE CABLE, THE END MICROPHONE BEING TERMINATED WITH RESISTORS SO THAT CABLE CUT BRIDGE CIRCUIT DETECTS AN UNBALANCE WHEN ANY MICROPHONE CABLE IS CUT TO PROVIDE AN ALARM SIGNAL AND REDUCE R-F INTERFERENCE.

SEAQOM u 3,553,687

M. H. REISS ALARM SYSTEM SENSITIVE TO CUT CABLES AND Jan. 5, 1971INSENSITIVE TO R-F INTERFERENCE Filed June 5 l Int. Cl. GOSf 29/00 U.S.Cl. 340-409 `ABSTRACT OF THE DISCLOSURE In an alarm system with abalanced shielded micro- United sta-'tes Pat-em o 11 Claims 1 phonecable a bare drain wire is tied to the cable shield l at the end of themicrophone cable, the end microphone being terminated with resistors sothat a cable cut bridge circuit detects an unbalance when any microphonecable is cut to provide an alarmsignal and reduce R-F interference.

BACKGROUND OF THE INVENTION The present/invention relates in general toalarm sys-v tems and more particularly concerns an acoustical alarmsystem of the type including an input transducer for sensing an alarmcondition having means for sensing that a cable from the sensingtransducerto the alarm indicating equipment has been cut.

Acoustical alarm systems, such as the Detectalarm syst tem made byAlarmtronics Engineering, Inc. perform well for the purpose ofindicating an alarm condition when a microphone receives an unfriendlynoise produced by an'intruder. Should Vvan unfriendly intruder somehowcut the cable.` joining the sensing microphone to the alarm indicatingunit, such as by gaining access to the'cable during the day when thealarm system is disabled, it is possible that such an intruder couldgain en- SUMMARY OF ,THE INVENTION Accordingto the invention, there istransducing means at a remote location for sensing an unfriendly signalsource and providing an electrical signal overa transmission cable tothe alarm condition monitoring unit.

The transmision cable includes a drain conductor con-` n nected to theend of the cable shield where the most remote transducer is located,this most remote transducer l 3,553,687 y Patented Jan. 5, 1971 'iccsensing transducer have a pair of equal resistors con nectedtherebetween with the junction of these resistors connected to theshielded conductor. The alarm unit end of the cable is coupled by a pinetwork to the primary of the input transformer that carries normalmicrophone sounds to the main channel amplifier. The primary of thistransformer is center tapped and coupled to a detecting circuit thatsenses a D-C imbalance whenever a cable is cut to provide an appropriatesignal that could be an -alarm signal. Twisted conductors and a groundeddrain wire coupled to a balanced input help reduce R-F interference.

Numerous other features, objects and advantages of the invention willbecome apparent from the following specification when read in connectionwith the accompanying drawings.

BRIEFA DESCRIPTION OFTHE DRAWING The single gure showsa schematiccircuit diagram of a preferred embodiment .of the invention.

DETAILED DESCRIPTION OF PREFERRED l EMBODIMENTS With reference now tothe drawing, there is shown a schematic circuit diagram of an exemplaryembodiment of `when an unfriendly noise is sensed. The output ofdetecting circuitry 22 will also cause alarm indicator 21 to indicate analarm condition when any of the cables have been cut.

Each of the microphones 14, 15 and 16 are the end microphones of cables11, 12 and 13. It is to be understood that additional lmicrophones maybe connected into the main cables 11, 12 and 13 at intermediate points,

` such as those designated by the letters A, B and C along the line, butthese microphones are not terminated with 'resistors as are inmicrophones 14, 15 and 16. The detailed description which follows shallbe only of elements associated with main cable #l because thoseassociated with cables `#2 and #3 are similar.

Each of the end microphones may be of a type having an input windingsuch as 31 with each end connected to one of the two twisted microphonesignal conductors, such as 32 and 33. Equal resistors, such as 34 and35, terminate the ends of winding 31 as shown with their junction 36connected to the microphone case 37 and to the bare drain wire 41 shownby broken line for clarity inside microphone cable 11 which in turn isgrounded v at 'the input terminal 42 at the alarm monitoring unit.

being terminated in impedance means such that cutting of the cableproduces an unbalance that is sensed by a detecting unit. Morespecifically, the transmission cable'is balanced two-wire shielded cableincluding a third conductor in side the shield that is connected to theshield at the microphone end and to ground at the unit end. The twotwisted signal wires that are connected to the remote This bare drainwire 41 is also vconnected to the cable shield 43 at the end 44 asindicated. Signal conductors 32 and 33 are connected by equal isolatingresistors 45 and 46, respectively, to R-F chokes 47 and 48,respectively, which in turn are connected to respective ends of theprimary 51 of transformer 52. The secondary 53 of transformer 52 iscoupled to the main alarm channel amplifier 17 to transmit soundsreceived by any of the microphones to alarm indicator 2l for analysisand provision of an alarm indication when the noise is unfriendly.

Primary winding 51 has a center tap 50 that is coupled by R-F choke tothe base of transistor Q1 and one end ofbalancing potentiometer 56 ofcable cut bridge -circuit 57. The bridge circuit 57 includes only threeoutside arms, the first beingbalancing potentiometer 56 in series withresistor 61, the second being resistor 62 and the third being resistor63. When transistor Q1 conducts, the series combination of itsbase-emitter resistance and resistor 64 is connected across opposedjunctions 65 and 66 is normally at` +-3 v. DC when the cables are notcut and the end microphones terminate the cable. However, if the cablebecomes cut, center tap 54 is no longer at a +3 v. DC potential. The-base of transistor Q1 then begins to rise toward the +6 supply voltageon terminal 67 at the said drain conductor being connected to saidshield at the end thereof near said transducing means. 3. An alarmsystem in accordance with claim 1 and further comprising,

junctions of resistors 61 and 62. Transistor Q1 then conat ducts, andits collector dropsin potential as current is drawn through collectorresistor 71. Since transistor Q2 PNP transistor Q2, previouslynonconductive because its emitter is coupled through emitter resistor 72to the +6 volt potential on terminal 67, conducts. The potential on thecollector of transistor Q2 then begins to rise as current flows throughits collector resistor 73 connected to the -4 volt potential on terminal74. This positive going potential rise is coupled by diode D1 to alarmindicator 21 to provide an alarm indication, thereby warning that thecable has been cut.

The invention thus allows virtually any practical number of microphonesto be connected to the monitoring unit while providing an indicationthat even one microphone signal conductor has been cut, for cutting anyone signal conductor willdestroy the balance and cause tran sistor Q1 toconduct. This `cable cut monitoring is` ac `complished reliably landwith a relatively few inexpensive componentsthat consume negligiblepower.

In sumary the invention detects cable cut, reduces vR-F interference andutilizes a pair of lines performing the dual functionof bothcarrying D-Ccable cut current and audio signal current. a

It is evident that those skilled in the art may now make numerous usesand modifications of and departures from the specific embodimentdescribedherein without departing from the inventive concepts.Consequently, the invention is to be construed asembracing each andevery novel feature and novelcombination of features present in orpossessed by the apparatus and techniques herein disclosed and limitedsolely by the spiritand scope of the appended claims.

The embodiments of the invention .in which an exclu v vhas its baseconnected to the collector of transistor Q1,

n 10 Potentiometer 56 is normally adjusted so .that tran-rv vsistor Q1is nonconductive. The base of transistor Q1 `alarm signal'monitoringmeans at the remote location having an input balanced with respect to areference f cable cut signal indica able havin been QL.

4. An alarm system in accordance with claim 3 wherein Asaid means forcoupling to .the latter balanced input comprises a pi resistive networkhaving first and second legs of substantially equal resistance in serieswith respective ones of said firstand second signal conductors.

sive property or privilege is claimed, aredefinedas follows:

-1. An alarm system comprising, i' transducing means for providing asignal representative of an alarm condition, cable means for couplingsaid transducing means to a ocation remote from said transducing means,said .cable means having first and second signal conductors, and a drainconductor, and first and second substantiallyA equal resistors con- 1nected in series across the input'to saidtransducing means, the junctionof said series-connected resistors being connected to said drainconductor, the other end of each of said resistors being connected 1 vtoa respective one of said first andsecond signal 5. `An alarm system inaccordance with claim 3 wherein said latter balanced input comprises atransformer having a tapped primary, the tap on said primary comprisingsaid intermediate terminal.

6. An alarm system in accordance with claim 5 wherei in said .detectingmeans comprises,

' a bridge having first, second and third resistive legs connected inseries between a first bridge junction and said reference terminal todefine asecond bridge junctionbetween said first and second legs and athird bridge junction between said second and third legs,

means for `D,C coupling said tap to said rst bridge junction,

a normally nonconductive first transistor having its base connected tosaid first bridge junction, its emitter D-C coupled to` said thirdbridge junction and its collector D-C coupled to a source of energizingpotential that is'connected to said second junction,

:jwhereby cutting of said cable causes said transistor tov becomeconductive.

7. An alarm system in accordance with claim 6 where 1n said cablecomprises said first and second signal con- .ductors forming a twistedpair inside an insulatedly separated conducting-shield also enclosingsaid drain conductor,

said drain lconductor being connected to said shield at the end thereofnear said transducing means. 8. An alarm system in accordance with claim7 wherem said means for coupling to the latter balanced input ycomprises api resistive network'having first and second legs ofsubstantially equal resistance in series with respective ones'of saidfirst and second signal conductors. 9. An'alarm system in accordancewith claim 3 and further comprising at least one other of saidtransducing means,

at least one other of said cables'forlcoupling said at l least one othertransducing means to the remote location, f `at least one other pair offirst and second substantially equal resistors connected in seriesAacross the input to said at least one other transducing means, thejunction of the latter series connected resistors being connected to thedrain conductor of said at least one other cable,

the other end ofeach of the\latter resistors being conand means forcoupling the latter rst and second signal f conductors.. to said alarmsignal'monitoring means balanced input.

10. An alarm systern in accordance with claim 9 wherein each of saidcables comprises said rst and second signal conductors forming a twistedpair inside an insulatedly separated conducting shield also enclosingsaid drain conductor,

said drain conductor being connected to said shield at the end thereofnear said transducing means.

11. An alarm system in accordance with claim 1 and further comprising atleast one other transducing means having first and second inputterminals connected respectively to said rst and second signalconductors respectively at a point on said cable spaced from the endthere- UNITED STATES PATENTS 1/1944 Cash et al 178-69 1/1965 Lamer 34e-409 DONALD J. YUSKO, Primary Examiner 10 C. M. MARMELSTEIN, AssistantExaminer U.S. Cl. X.R.

